The principal investigator has shown the neutropenia is associated with increased serum levels of colony stimulating factor (CSF).In vivo studies with various preparation of purified CSF suggest that this factor may be a granulopoietin. Moreover, diffusion chamber granulopoiesis is markedly inhibited with an antibody directed against CSF. These ongoing studies will purify large quantities of murine L-cell CSF and determine the effect of this factor in normal and myelosuppressed mice. Experiments will also study the effect of a pure CSF antibody in vivo. Pure CSF will be analyzed for amino acid content and, following trypsin digestion, studies will determine whether there are low molecular weight biologically active peptide sub-units. The purified CSF and anti-CSF will be used to identify CSF producing cells by immunofluorescent techniques. Radioiodinated CSF will be used for radioautographic studies to identify and characterize CSF-responsive cells (granulocytic stem cells). Radioiodinated CSF and high titer antiserum will be used to design a radioimmunoassay for this factor. With appropriate modifications this should prove useful in the study of human granulopoiesis.